SUN TRACKER

SUN TRACKER

Due to the increasing demand for energy in the world, there is increased pressure to not only come up with new sources of energy but also to increase the efficiency of existing ones. Solar energy is an attractive option for tackling this energy challenge since it is an abundant and green source of energy whose potential is far from being completely utilized. Among the challenges limiting the efficiency of harnessing its energy include its fluctuations throughout the seasons and its absence at night. Of concern in this article however is its movement across the sky, which limits the efficiency of stationary solar harnessing devices like solar panels that point only in a specific direction.

This challenge necessitates the use of a sun tracker which is basically a device that orients or aligns a solar energy harnessing device like a solar panel, mirror, lens or a parabolic trough towards the sun. It changes its orientation throughout the day as shown in Figure 1, in order to follow the sun’s path, thereby maximizing the energy capture.

Figure 1: Sun tracking set up

Why track the sun?
Studies have shown that solar cells harness up to 40% more energy from the sun when they track it across the sky [1]. The sun has two main components namely the direct beam and the diffuse sunlight. The direct beam carries approximately 90% of the solar energy whereas the remainder is carried by the diffuse sunlight. It therefore goes without saying that it is important to ensure that the direct beam shines as long as possible on the solar panel. A sun tracker aims to realize this by reducing the angle of incidence between the incoming rays of the sun and the solar panel

Categories of sun trackers

Sun trackers can be categorized as follows

based on the source of their energy

Active sun trackers

These trackers have to be fed with energy, mostly from electricity, since they rely on motors, hydraulics and gear trains. They have been found to be more efficient that their passive counterparts especially at low temperatures. They can be microprocessor and electro-optical sensor based, auxiliary bi facial solar cell based, PC controlled date and time based and a combination of the three aforementioned systems [2].

Passive sun trackers

These do not require an extra source of energy and are mostly based on compressed fluids that boil at low temperatures. They also include shape memory or Freon and though less inaccurate, they have an advantage of having simpler designs.

Based on the number of axis

Single axis: This can only be adjusted on one axis only as shown in Figure 1.

Figure 2: Single axis solar tracker [3]

Double axis: These can be adjusted on two axes as shown in figure 2.

Figure 3: Dual axis solar tracker [3]

Design of the sun tracker

Our design aim is to get the best from both worlds by combining the high efficiency of active trackers with the simple design of passive trackers [4].

It is based on an arrangement of solar cells or panels connected to a reversible direct current (DC) motor. In order to simplify the design, the solar cell does the tasks of both sensing and providing the energy required for tracking. The sun tracker contains a collector, tracking and backtracking cells, a motor, a rotor and a stator (stand). The setup is assembled as shown in Figure 4 whereas Figure 5 shows the setup before sunrise. One solar cell is fixed to the tracker’s rotary axle with its plane inclined at an angle β around 20° eastwards as shown in Figure 6. It is also set up such that it is perpendicular to the solar energy collectors.

Figure 4: Assembled sun tracker [4]

The solar cell is mounted on the rotor, which is also connected to the DC motor. To ensure that the tracker remains at a specific point until the motor is activated to track the sun to the next position, a self-locking transmission is used at the axle.

At sunrise, the solar collectors are eastwardly oriented and as the sun rises and starts moving to the west, the incidence angle β increases on the sensing or driving cells. This increases the power of the driving DC motor up to a point (threshold point) where it is enough to move the solar collectors. This movement happens until the power of the DC motor falls below the threshold position. It will therefore follow the suns path, being in different positions during the day as shown in Figure 7. In order to enable backtracking from any position, additional panels are mounted in an antiparallel manner thus making the tracker compact, cheaper and more reliable than preceding trackers which use sensing solar cells either separated by shadowing or which are perpendicular to each other.

The tracking angle is maintained at approximately 120° and not more since beyond this angle, there is no substantial increase in the collectible energy with increase in the tracking angle. In order to cater for both circumsolar and isotropic diffuse components in the sunlight, the collectors have a plus or minus 10° tolerance within which there is no reduction in collected energy. The stand can be fabricated from Aluminium and stainless steel.

Figure 6: Tracker before sunset [4]

Figure 7: Tracking during the day [4]

Advantages of the sun tracker

Simplicity in design.

Works well at low temperatures.

Enables backtracking from any position.

It is cost effective by eliminating expensive electronics associated with other trackers